1. Field of the Invention
The invention relates generally to the field of seismic surveying of the Earth's subsurface. More specifically, the invention relates to methods for determining whether seismic data have been acquired to sufficient spatial density to avoid distortions in generating images of the Earth's subsurface from seismic data.
2. Background Art
In seismic surveying, seismic energy sources are used to generate a seismic signal that propagates into the earth and is at least partially reflected by subsurface seismic reflectors. Such seismic reflectors typically are located at the interfaces between subterranean formations having different acoustic properties, specifically differences in acoustic impedance at the interfaces. The reflections are detected by seismic receivers at or near the surface of the earth, in an overlying body of water, or at known depths in boreholes. The resulting seismic data may be processed to yield information relating to the geologic structure and properties of the subterranean formations and their potential hydrocarbon content.
A purpose for various types of seismic data processing is to extract from the data as much information as possible regarding the subterranean formations. In order for the processed seismic data to accurately represent geologic subsurface properties, the reflection amplitudes need to be represented accurately. Non-geologic effects can cause the measured seismic amplitudes to deviate from the amplitude caused by the reflection from the geologic target. Amplitude distortions resulting from irregular distribution of source and receiver positions during data acquisition is a particularly troublesome non-geologic effect. If uncorrected, these non-geologic effects can distort the seismic image and obscure the geologic picture.
A seismic energy source generates an acoustic wave that reflects from or “illuminates” a portion of reflectors at different depths in the subsurface. In a three-dimensional (3D) survey, seismic signals are generated at a large number of source locations, detected at a large number of receiver locations and the survey generally illuminates large areas of the reflectors. U.S. Pat. No. 7,336,560 issued to Rekdal et al. describes certain data density issues concerning marine seismic data. According to the Rekdal et al. '560 patent, processing techniques known in the art including prestack 3D migration algorithms can produce good images of the sub-surface horizons only if the surface distribution of sources and receivers is relatively uniform. In practice, there are typically irregularities in the distribution of sources and receivers. Obtaining perfectly regular acquisition geometry is typically impracticable. Consequently, according to the Rekdal et al. '560 patent, prestack 3D migrated seismic images often include non-geologic artifacts. Such artifacts can interfere with the interpretation of the seismic image and attribute maps.
In marine seismic surveying, one or more sensor cables called streamers is towed by a survey vessel near the surface of a body of water. A seismic energy source such as an air gun or air gun array is actuated at selected times. It is well known in the art that in marine seismic surveys, the streamers generally do not form straight lines behind the survey vessel. Typically marine currents and other factors such as propeller wash from the survey vessel cause the streamers to be displaced laterally, a phenomenon called “feathering.” Changes in marine currents often cause changes in the feathering. In such circumstances, if the planned sail line (direction of motion) separation of the seismic vessel is maintained, then feathering will lead to coverage “holes” at some offsets or offset ranges. The term “coverage hole” as used in the Rekdal et al. '560 patent refers to a surface area where, for a given offset (source to sensor distance) or offset range, there are believed to be inadequately spatially sampled data recorded. Data are typically defined to be “located” at the surface midpoint positions between the seismic source position and the seismic sensor position at the time of acquisition of a seismic signal recording. Such coverage holes can vary in size, irregularity, and density of data remaining in the hole. It is possible to have holes where no data are present. Coverage holes may be of several kilometers extension in the sail line (inline) direction where streamers have feathered in the same direction for a long continuous length of the intended sail line, but are generally smaller in the crossline direction (orthogonal to the sail line), as this width is governed by the amount of feathering of the streamers.
In marine seismic streamer surveys, if data density criteria known in the art are used, portions of the subsurface may be believed to be inadequately covered with seismic data recordings due to streamer feathering and other causes. Thus, using such prior art seismic data density evaluation criteria, it may be believed that additional passes of the seismic vessel and streamers through the prospect survey area are required. Additional “sail-lines” (passes of the vessel and streamers through the survey area) were also thought to be needed by reason of steering the vessel to achieve acceptable coverage. That means that the lateral distance between streamer positions in all the passes made by the vessel can be on average less than in the original acquisition plan. These additional passes significantly increase the time and associated cost to complete a survey. These additional passes of the survey vessel are referred to as “infill shooting” or just “infill.” A large portion of marine seismic data acquisition in a particular survey area can be infill shooting because of perceived inadequacy of data density. The infill shooting may take up to several weeks or even months to complete. Thus, it is not uncommon to spend 15-30% of total acquisition costs on infill acquisition.
According to the Rekdal et al. '560 patent, the maximum data hole sizes that will provide acceptable subsurface coverage are typically determined prior to acquisition, and are typically independent of local factors such as geology and survey objectives. Criteria for a seismic survey, such as acceptable subsurface coverage, are commonly called “infill specifications.” An object of the method described in the Rekdal et al. '560 patent is to determine whether the coverage holes are of sufficient size so as to require infill acquisition.
The method disclosed in the Rekdal et al. '560 patent, as one example, makes use of certain assumptions about the required degree of data coverage based in part on substantially discontinued seismic data processing procedures. Such procedures, for example, consisted of “binning” the acquired seismic data, summing or “stacking” seismic data within each bin, and then “migrating” the data after stacking. The requirements for migration in such processing are that each of the stacked traces reasonably represents the same sum of a set of offset traces at each location. In order for the stacked trace to have similar properties at each location associated with a bin, it is important that the stacked trace be the sum of a set of similar “offset” (distance between the seismic source and receiver) traces.
To ensure such similarity, traces are summed over a small area (a “bin”) such that a contribution from each of the expected offset traces is present in the sum. There are several problems with such procedure. First, the traces are summed over an area. Even if normal moveout (“NMO”) has been correctly performed, in the presence of reflective horizon “dip” (change in depth with respect to position), the reflective event times will not be aligned. This is often referred to as “bin smear”, and results in the loss of high frequency data content for dipping reflective events. Second, if a trace at a particular offset is missing, then either new data should be acquired (infill data), or the bin can be expanded (overlapped into adjacent areas) to see whether a suitable trace is available. Such bin “flexing” obviously increases the “bin smear”, but if only a small number of traces are used, this may not be a large problem. If an acceptable trace is found, then it is copied into the required bin and may therefore now contribute to more than one stacked trace.
Some bins may contain more than one trace of the required offset. In order to keep the stack trace balance similar at all bin locations, extra traces in any such bin are not used. There are several criteria for which trace of a plurality of traces in a bin should be used, but most commonly the trace that is selected is the one having a position closest to the position of the bin center, as this potentially reduces the bin smear. However, such procedure means that some of the traces that have been acquired may be discarded from further processing.
It is currently common in seismic data acquisition, as explained above with reference to the Rekdal et al. '560 patent, to make decisions on whether infill data should be acquired based on an evaluation of what traces fall in each bin of the survey. A procedure known as “flex binning” may be performed (typically in real time during acquisition) to infill “holes” where some offsets are missing from certain bins. However, it is uncommon to “flex” more than a small distance either side of the nominal bin location because of the bin smear that would be associated with collecting traces from further away, and such “flexing” is usually based on a rectangular bin criteria.
It is known in the art to perform migration on seismic data prior to stacking. See, for example, U.S. Pat. No. 6,826,484 issued to Martinez et al. In a prestack migration sequence, each trace to be processed is migrated using its actual location (not the average of a stack set, or a theoretical bin center). Trace locations may be output from the migration stage at any selected location, and such locations are generally positioned on a grid which is associated with bin centers. The output traces can then be stacked. Despite the change in processing methodology from post stack migration, the traces selected for processing and the methods of infill selection used in the industry have remained essentially the same.
The assumptions concerning data coverage as explained above have caused the development of marine seismic survey techniques in which is it is desirable to maintain the geometry of the streamers as closely as possible in a straight line, parallel pattern behind the survey or towing vessel. There are devices known in the art for steering seismic streamers, and methods for using such devices have been developed that have as an objective the arrangement of streamers in such straight, parallel patterns despite factors such as propeller wash from the survey vessel and cross currents in the water (transverse to the direction of motion of the survey vessel). See, for example, U.S. Pat. Nos. 6,932,017, 7,080,607 and 7,162,967 issued to Hillesund et al. with reference to streamer steering methods and systems. An example streamer steering device is described in U.S. Pat. No. 6,144,342 issued to Bertheas et al.
There continues to be a need for marine seismic acquisition techniques that reduce the amount of infill coverage and increase overall survey efficiency.